I have an Onkyo 602 AV receiver and it does the same thing. As mentioned above you can lock in the audio format for each input signal, if you don't it is factory set to "auto" which will cause the receiver to "click" as it changes to the different audio signal inputs it sees. Watch the display screen on the receiver, every time you hear it click you'll notice the display will change to reflect the new input signal it's seeing. I notice it a lot when I'm watching a show that is broadcast in Dolby and then when it goes to a commercial it will switch to stereo. So there is nothing really wrong, it's just the "auto" feature switching around to match the input signal.